1. Field of the Invention
The invention relates to glass, ceramic and metal substrates with at least one structured hydrophobic surface which provides a good self-cleaning effect. Another subject of the invention is a method of making said substrates with a self-cleaning surface, the method comprising forming a structured surface then making it hydrophobic. A further subject is use of the glass, ceramic and metal substrates with a surface according to the invention having a self-cleaning effect.
It is known that in order to obtain a good self-cleaning effect a surface needs to have not only good hydrophobic properties but also a micro-rough surface structure. Both features are present in nature, for example in the lotus leaf; the surface, formed from a hydrophobic material, has pyramidal elevations a few micrometers away from each other. Drops of water substantially come into contact only with these tips, so the contact area is minute, resulting in very low adhesion. These relationships and the theoretical possibility of applying the “lotus effect” to industrial surfaces are taught by A A Abramzon, Khimia i Zhizn (1982), No. 11, 38-40.
2. Description of the Related Art
Without reference to the lotus effect, water-repellent surfaces are known from U.S. Pat. No. 3,354,022, where the surface has a micro-rough structure with elevations and depressions and is formed from a hydrophobic material in particular a fluorine-containing polymer. In one embodiment a surface with a self-cleaning effect may be applied to ceramic bricks or to glass by coating the substrate with a suspension containing glass spheres with a diameter within the 3-12 μm range and a glass sphere (diameter 3-12 μm) and a fluorocarbon wax based on a fluoroalkylethoxy methacrylate polymer. A disadvantage of such coatings with a self-cleaning effect is their poor resistance to abrasion. As established by the inventors involved in this patent application, glass spheres do indeed form a structure but their self-cleaning effect is only moderate.
EP 0 909 747 A1 teaches a method of producing self-cleaning properties in surfaces, particularly roof tiles. The surface has hydrophobic elevations 5 to 200 μm high. A surface of this type is formed by applying a dispersion of powder particles of an inert material in a siloxane solution then letting it harden. As in the previously assessed method the structure-forming particles are not fixed on the surface of the substrate in an abrasion-resistant manner.
EP 0 772 514 B1 and EP 0 933 388 A2 teach of self-cleaning surfaces on articles with an artificial surface structure comprising elevations and depressions, the distance between the elevations being within the 5 to 200 μm range (EP 0 772 514 B1) or the 50 nm to 10 μm range (EP 0 933 388 A2) and the height of the elevations being within the 5 to 100 μm range or the 50 nm to 10 μm range respectively and the structure being made of hydrophobic polymers or materials made durably hydrophobic. Methods suitable for forming the structures are etching and embossing processes, coating processes for sticking on a structure-forming powder and shaping processes using appropriately structured female moulds. If the material forming the structure is not itself hydrophobic the formation of the structure is followed by treatment to make it hydrophobic, particularly by silanising it. Although self-cleaning surfaces according to EP 0 772 514 B1 may also be applied to glazing or roofs the process is very expensive and the surface forming the structure, like that in the documents assessed above, has little resistance to abrasion, so the self-cleaning effect declines rapidly under quite strong mechanical stress.